Patricia Manning's brother Osman Cameron died in the Queen Elizabeth psychiatric hospital in Birmingham two weeks ago, and she has been tormented by what-ifs ever since. What if she had gone to visit him earlier that morning when she found out her college lecture had been cancelled? What if she hadn't stopped to pick up some snacks on the way to the hospital? What if he were still alive?

Osman, who was 45 and didn't work, was found dead in his hospital room on January 24. He had been arrested on January 4 after getting into an argument with a postman outside his home, and taken to the Belgravia police station in Birmingham, where he spent two nights. According to West Midlands police, he was assessed by doctors and social workers and a decision was made to section him under the Mental Health Act. He was transferred to the hospital on January 6. He died 18 days later, three days before he was due to be released.

It is too early to say with any certainty how Osman died, but the preliminary results of a postmortem appear to suggest asphyxia, possibly as a result of an epileptic fit. The inquest, opened at Birmingham coroners' court last month, is due to resume later this month.

It's a terrible tragedy for the family, who argue that Osman was a vulnerable man who suffered from autism and epilepsy and who should never have been in a psychiatric hospital. But for Patricia and her sister Elizabeth Melbourne, Osman's death is a doubly cruel blow. Just over nine years ago, on December 8 1995, their other brother, Alton Manning, also died in the care of the state, this time in Blakehurst prison in Worcestershire. (Despite their surnames, the two were full brothers.)

The circumstances of Alton's death were rather different. On remand for an alleged violent offence, he was selected by prison guards for a strip search, to which he succumbed without protest. When they ordered him to squat for an anal and genital examination, however, he refused, and a struggle ensued. The inquest into his death heard that Alton was thrown to the floor and held face down while officers held his head and legs. He was taken out of the cell and carried face down along the corridor. Prisoners told the hearing that they saw him being held in a neck hold, a procedure known to be extremely dangerous.

He was then placed face down on the floor (other prisoners stated that they saw officers knock him to the ground). One said that what he saw "frightened the daylights out of him". Officers noticed a pool of blood near Alton's mouth and at this point his body went limp. The officers called for medical help, but continued to restrain him. By the time the nurse arrived, he was dead. He was 33.

A postmortem found that Alton died of asphyxia related to the way he was restrained. He had bruising to the neck and back, blood spots in the eyes, face and neck, blood from the ear and mouth, eight separate injuries to the face, and abrasions to the arms and legs. None of the officers sustained any significant injury, despite claiming he had resisted violently.

In the years that followed Alton Manning's death, his family campaigned tirelessly to bring the prison officers they believed responsible for his death to justice. Buoyed by an unlawful killing verdict at an inquest into his death in 1998, they hoped and believed that a prosecution would follow. None did. The Crown Prosecution Service argued there was "insufficient evidence" to proceed. When court proceedings brought by the family forced the CPS to reconsider in 2002, they again refused. After seven years, the family were forced to concede defeat.

Alton's death affected the Manning family in unimaginable ways. Patricia's father Cleveland, a carpenter, who was in remission from cancer, fell ill on the day of his son's death. Six weeks later he was dead. Her mother Albertha slipped into depression and died in Jamaica in 1998, two weeks into a holiday it had taken the sisters six months to persuade her to take.

"Anyone who knew Alton would say he was a proper gentleman," says Patricia. "Not a hair out of place. You know, like one of those army people? Shiny shoes and everything. My parents died of heartbreak. We were a close family. When my brother died, the shock immobilised the use of my father's legs and he was taken to hospital that day. As far as I'm concerned, the cancer began growing on the same day. My sister and I alternated looking after my mother, trying to encourage her to come on. But there was nothing we could do. She'd lost her husband and her son." She is buried in Jamaica, in the garden of the house where she was born.

She says it was only anger at the way her brother died that kept her going. "It was a traumatic time, to say the least. I think for me and my sister, the adrenaline because of the anger that you felt and because of the physical marks that he had, somehow gave you the strength to cope, even though my family were dropping like flies. We seemed to be able to deal with it."

To lose a second brother, however, one for whom she felt an almost parental responsibility, has proved much harder. Patricia was told that the police were called after Osman got into an argument with a postman outside his flat in Sparkbrook, Birmingham, apparently over the fact that a letter he was expecting had not arrived. She was not told that he had been arrested until two days later, when Osman's social worker got in touch to let them know that he was to be sectioned. She visited him in the hospital as often as she could, and called him when she couldn't.

Patricia talks about her brother, affectionately known as Ossie, with enormous tenderness and pride. In the way a mother would recount the achievements of her child, she lists his triumphs. How he nearly played table tennis for the junior England team. How musical he was, playing the recorder, the guitar and the organ. How intelligent he was, managing to stay on at mainstream school despite his autism. How he tidied friends' gardens to keep himself occupied.

But she knows this wasn't always the way strangers saw him. Intensely shy, Osman found it difficult to make conversation and Patricia admits he could come across as strange. "He was a bit of a loner," she says. "He always carried a big shopper bag with all his things in, his recorder, his books. To some people he probably appeared stupid, slow, thick. He was an easy target and people used to pick on him. People would judge him just on his appearance."

As a result, he was often getting into trouble. Patricia lost count of the number of times he was brought home or taken into custody by police after an argument on a bus or on the street. Whether as a result of his experiences or as a consequence of his condition, he didn't like going into new situations or into areas where people didn't know him.

According to Alan Bicknell of the National Autistic Society, this kind of behaviour is common amongst autists, who tend to have problems relating to and empathising with other people. "Everyone is different, but autism can result in extreme shyness," he says. "There can be problems with communication, with diction and the volume of the voice, which means people can come across as being loud and odd. There can also be problems with flexible thinking - new people and places - and accepting that other people think differently from them. They can find it virtually impossible to read body language."

It is not unusual, he says, for autists to attract the attention of the police. "We see that situation only too often," he says. "The police want a person who has been stopped to show them respect. But someone with autism will avoid eye contact and appear unconcerned. There is no obvious physical disability, so it won't necessarily be obvious to an officer that anything is wrong."

But Patricia believes there was another dimension to the attention Osman attracted, which began after he left school. "It was the 70s, he was black," she says. "He had a lot of problems with the police. Sometimes they'd arrest him, sometimes they put him in a psychiatric hospital, sometimes they wouldn't even arrest him, they'd just bring him home. I'm not saying all of them were bad. There was one that used to come to the house and speak to my mum because he was really concerned about Ossie." It was not until three years ago, after her parents had died and his care fell to her and her sister, that Ossie was diagnosed as autistic.

"When he was diagnosed, everything clicked into place," she says. "It made sense, the routine, the fact that he sometimes didn't understand your reasoning, the fact that he hated change." But his diagnosis didn't always help with the police. "I hate having to bring colour into it," she says. "But I can't help wonder, would the circumstances have been the same if the police had picked up a young, white autistic male who was having an altercation with somebody in the street? Rather than treat him as a criminal, maybe they would have seen him as someone with a problem who needed help."

She is deeply hurt by the release of a statement by West Midlands police immediately after Osman's death which claimed he had been arrested for robbery. The force has since changed the statement, but the inaccuracy was picked up and repeated unquestioningly by local and national media. "I don't know where that came from, because the hospital, the social worker and I were all told it was an altercation," she says.

Osman's death is now the subject of an investigation by West Midlands police, who will produce a report for the Birmingham coroner. The force voluntarily referred the case to the Independent Police Complaints Commission, but it was sent back to them after the IPCC concluded that there were "no identifiable issues of concern" relating to his death.

Of six children born to their parents, only Patricia and her sister Elizabeth, who also lives in Birmingham, survive. Their brother Barrington died aged four of asphyxia related to an asthma attack; another sister was stillborn.

She struggles to explain how it feels to lose two brothers in custody. "People say lightning doesn't strike twice, but look at what has happened to my family. This time was definitely the worst. I couldn't even leave the house. And then having to explain it to the children. I suppose it brought back my feelings about Alton. To lose another family member in those circumstances, it's even harder. It tears families apart. I just want people to discard the stereotypes and know that he is a person. He had nieces and nephews, he loved life, he had a right to life. Ossie was a very loving man who will be missed. Who will be missed terribly."

· To find out about the National Autistic Society's campaign to raise awareness of the condition among criminal justice professionals, visit www.autism.org.uk/cjp